Many modern day internal combustion engines are designed to run with fuel/air mixtures that are both economical and non-polluting. Recently, in order to meet emission standards fuel/air ratios have become leaner resulting in reduced pollution but sometimes also resulting in degraded engine performance.
One of the problems with the so-called "lean burn" engines is that the low fuel/air ratio is difficult to properly ignite. The standard two electrode spark plug with its small gap is a very inefficient ignitor for the lean ratios because the effective ignition surface is only the highly ionized area of the spark itself, which is quite small.
Various arrangements have been devised to overcome the poor ignition characteristics of lean fuel/air mixtures. One of these arrangements involves using a small pre-ignition chamber in which a much higher fuel-to-air ratio is introduced to obtain initial ignition, the flame front then spreads from the pre-ignition chamber into the main cylinder area where it ignites the main fuel/air mixture.
Other attempts have been made to change the characteristics of the spark plug or ignitor mechanism itself. One of the most successful variations of the normal spark plug arrangement is known as a plasma jet ignitor. An ignitor of this type is shown in U.S. Pat. Nos. 3,906,919 and 3,842,819 and usually consists of an insulator and housing similar to an ordinary two-electrode plug. However, a cavity is formed in the alumina insulating material at the end where the usual spark gap is located. At one end of the cavity a central electrode is located. The other end of the cavity is closed by a metal plate having a small orifice located near the center of the cavity. When the plasma jet ignitor is used with a standard reciprocating piston engine during the compression cycle, a small amount of the fuel/air mixture is forced through the orifice into the cavity formed in the insulating material. At the proper time, a high voltage is applied to the central electrode causing an arc to form between the central electrode and the orifice plate. This arc initiates formation of a highly ionized plasma in the cavity which, due to the expansion of the plasma, spews forth from the orifice into the main cylinder area causing a large plume. The plume has sufficient area to ignite the main fuel/air mixture even in the case of lean mixtures.
Although the plasma jet ignitor is superior to the standard two electrode spark gap ignitor, it has several drawbacks. One of these is that, under normal operating conditions. Only a small amount of fuel is forced into the plasma cavity resulting in the small plume length in the main cylinder area. Attempts have been made to overcome this problem by introducing a small amount of liquid fuel into the plasma cavity through a secondary port passing through the insulating material of the spark plug. While the introduction of a small amount of liquid fuel does enhance the plasma plume length, there are practical problems in providing an access port through the spark plug insulating material. The presence of liquid fuel in the cavity area also produces dangerous vapors creating the danger of explosion.
Another problem with prior art plasma jet ignition devices is that a very high voltage is often needed to sustain the initial arc which starts plasma formation in the plasma cavity. Thus, the plasma jet ignition devices typically operate at two to three hundred times the voltage required to fire standard two-electrode gap spark plugs.
Accordingly, it is an object of the present invention to provide a plasma jet ignition device which has increased plume length over prior art spark plugs.
It is another object of the present invention to provide a plasma jet ignition device which is capable of firing with much lower voltages than prior art ignition devices.
It is a further object of the present invention to provide a source of solid fuel in the plasma cavity to enhance plasma plume length.
It is yet another object of the present invention to provide multiple spark gaps in the plasma cavity to reduce the firing voltage needed to operate the plasma jet ignition device.